The present invention relates to a nanostructural substance, and more particularly, to a nanostructural substance composed of primary particles (nanoparticles) having a small particle size and having a small contact area between the primary particles. In particular, titanium oxide particles constituted of a nanostructural substance of titanium exhibit a high catalytic activity and can be suitably used, in particular, as a photocatalyst.
When particles are finely divided into nano-order particles, the percentage of molecules occupying the surface of the respective particles becomes extremely large, so that the nano-order particles show a quite different properties from those of bulk materials. In particular, it is known that titanium oxide particles having a nano-order particle size exhibit a high activity. Therefore, it has been attempted to utilize such properties of the titanium oxide particles in various applications such as catalysts, cell or battery materials, magnetic materials, electronic circuit devices and biological materials. However, such nanoparticles tend to be strongly agglomerated together due to very fine particles and to be hardly dispersed in a monodisperse condition, thereby failing to fully exhibit properties thereof. In particular, the titanium oxide particles having a photocatalytic activity have a strong agglomerating force due to fine particles, so that it may be difficult to disperse the titanium oxide particles to such an extent as to exhibit sufficient functions thereof.
Hitherto, although various techniques for dispersing nanoparticles have been reported, many of these techniques relate to the dispersion and fixing of the nanoparticles onto the surface or into the inside of a third solid phase, e.g., an inorganic carrier such as alumina or a polymeric carrier such as resins.
For example, in Japanese Patent Application Laid-Open (KOKAI) No. 2003-112925, there is described a method of bonding surface-modified molecules having a molecular weight of not more than 1000 to magnetite nanoparticles to improve a dispersibility of the nanoparticles in an organic solvent; applying a dispersion of the nanoparticles onto the surface of a substrate; and then drying the dispersion applied to disperse and fix the nanoparticles thereon.
In Japanese Patent Application Laid-Open (KOKAI) No. 2003-297617, there is described a method of rapidly dropping a dispersion of nanoparticles synthesized by a reversed micelle method onto a substrate and drying the dispersion to form such a structure in which the particles in a nanometer scale are arranged in a self-organizing manner.
In Japanese Patent Application Laid-Open (KOKAI) No. 10-208236 (1998), there is described a method of dispersing and fixing nanoparticles of cobalt ferrite within an ion-exchange resin. Also, in Japanese Patent Application Laid-Open (KOKAI) No. 2001-316501, there is described a method of dispersing and fixing nanoparticles in a polymer having a crosslinked structure.
The above conventional methods of dispersing the nanoparticles require fixing of the nanoparticles to the third solid phase or in the presence of the third solid phase. However, in these methods, the nanoparticles may fail to be dispersed in a monodisperse state, and since a large portion or a part of the surface of the respective nanoparticles is covered with the third solid phase, it may be difficult to exhibit the performance of the nanoparticles to a maximum extent. Further, the third solid phase is inevitably required for dispersing the nanoparticles, resulting in limited applications thereof as well as complicated production process.
On the other hand, there are also known methods of producing high-functional particles by controlling a pore volume or a BET specific surface area thereof (Japanese Patent Application Laid-Open (KOKAI) Nos. 8-281060 (1996), 11-349328 (1999), 2000-203810, 2001-342010, 10-230169 (1998), 2000-191325 and 2001-114519, and PCT Pamphlet WO 99/1574).
Further, As one of methods for producing inorganic compound particles, there is known a spray thermal decomposition method.
In the spray thermal decomposition method, a raw material solution is sprayed through a nozzle or by irradiation of ultrasonic wave to form fine droplets, and then a solvent contained in the fine droplets is evaporated and thermally decomposed to obtain aimed particles.
Conventionally, there is also known the spray thermal decomposition method for producing fine oxide or metal particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 2001-342010 and 5-139738 (1993)).
Although at present, it has been most demanded to provide a nanostructural substance in the form of nanoparticles which are less contacted with each other and exhibit a good handling property, such a nanostructural substance has not been obtained until now.
That is, in Japanese Patent Application Laid-Open (KOKAI) No. 8-281060 (1996), there are described alumina, titania and zirconia having a large BET specific area value. However, primary particles of these materials may fail to have a sufficiently small particle size.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 11-349328 (1999), there are described titanium oxide particles whose agglomerated particles have an average particle diameter of 0.1 to 10 μm and whose primary particles have an average particle diameter of 10 to 1000 nm. However, the primary particles may fail to have a sufficiently small particle size. Further, although there are also described titanium oxide particles whose agglomerated particles are well controlled in average particle diameter, the titanium oxide particles are still small in BET specific surface area and, therefore, may fail to attain a sufficient contact efficiency.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 2000-203810, there are described hollow oxide particles having a shell thickness of not more than 20 nm. Although the specific surface area of the particles is increased by changing a surface condition thereof, the control of size of the primary particles is not taken into consideration in this Japanese KOKAI.
In addition, although Japanese Patent Application Laid-Open (KOKAI) No. 2001-342010 describes inorganic hollow particles, the aim of the invention of this Japanese KOKAI is to obtain hollow particles having a dense coating layer thereon, and therefore, the control of size of the primary particles is not taken into consideration in this Japanese KOKAI.
Further, although Japanese Patent Application Laid-Open (KOKAI) No. 5-139738 (1993) describes hollow zinc oxide particles, primary particles thereof tend to have a large particle size and a large contact area therebetween.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 2003-19427, there is described a method of producing fine particles by spray thermal decomposition method in which an inorganic compound other than raw materials is dissolved in a raw material solution. In the method of this KOKAI, a low-melting substance is used as the inorganic compound, and the heating temperature is set to not less than a melting point of the inorganic compound used. Under such a condition, crystal nuclei produced by thermal decomposition of the raw materials are contacted with the inorganic compound in a liquid state to allow the inorganic compound to exist between a plurality of crystal nuclei produced in the respective droplets, thereby forming fine primary particles. Therefore, in this method, the nanostructural substance in which the primary particles are present in a point contact with each other is not taken into consideration.
Furthermore, in Japanese Patent Application Laid-Open (KOKAI) No. 10-230169 (1998), although there are described titanium oxide particles having a crystallite size of 6 to 20 nm, the control of particle configuration of the titanium oxide particles is not taken into consideration. In Japanese Patent Application Laid-Open (KOKAI) No. 2000-191325, although there are described titanium oxide particles which are controlled in particle diameters of primary particles and agglomerated particles thereof, the control of particle configuration of the titanium oxide particles is not taken into consideration. In Japanese Patent Application Laid-Open (KOKAI) No. 2001-114519, although there is described titanium oxide which has a small crystallite size and is well controlled in pore volume, the control of particle configuration of the titanium oxide is not taken into consideration. Further, in PCT Pamphlet WO 99/11574, although there are described hollow titanium oxide particles, titanium oxide particles composed of fine primary particles are not taken into consideration.
As a result of the present inventors' earnest study for solving the above problems, it has been found that by sintering fine primary particles at points with each other to form spherical-shaped particles or a thin film-shaped material, it is possible to obtain a nanostructural substance which is easily handled without deactivation of properties inherent to nanoparticles. The present invention has been attained on the basis of this finding.
Thus, according to the present invention, there is provided an innovative technique capable of realizing a good dispersion of the objective nanoparticles by themselves without requiring a third solid phase.